Lac de Sylans

Lac de Sylans is a lake in the Ain department of the Auvergne-Rhône-Alpes region in France, located within the commune of Le Poizat-Lalleyriat, a few kilometres east of Nantua and roughly 20 kilometres south of Oyonnax. It ranks among the larger natural lakes of the Haut-Bugey area and draws both hikers and anglers.

Where exactly is the lake located?

The lake stretches along a valley in the Jura range, near the RD 1084 road connecting Nantua and Poizat. It covers about 49.7 hectares, measures close to 2 kilometres in length and up to 250 metres in width, with a maximum depth of 22 metres. Water levels drop noticeably in summer, when part of the inflow drains underground toward the village of Les Neyrolles.

How did the lake form?

The lake owes its existence to a landslide that collapsed part of the northern cliff of the Nantua cluse, blocking the outflow of the Doye springs and causing water to back up in the valley. Although its formation was long attributed to the early 17th century, historical evidence points to a much earlier origin: a papal bull issued by Pope Eugene III in 1145 confirms the privileges of Nantua Abbey over "lacum Silani" — the lake of Sylans — indicating it already existed in the 12th century.

What was the Sylans ice trade?

During the 19th century, the lake became the centre of a significant ice-harvesting industry. In 1864, Joachim Moinat, owner of the Café du Paradis in Nantua, began using ice cut from the lake to chill drinks for his customers. What started as small-scale harvesting grew into an organised operation producing an average of 300,000 tonnes of ice per year by 1880. The business passed to the Glacières de Paris company in 1885. A run of mild winters between 1911 and 1913, followed by the First World War, brought production to an end in 1917. The ruins of the old ice houses still stand on the lakeshore and remain open to visitors year-round.

Is the lake protected?

The area around Lac de Sylans has held classified-site status ("site classé") since 14 June 1909. In 2015, the Ain department granted it further protection as an ENS (Espace Naturel Sensible, or environmentally sensitive area), recognising the ecological value of its surroundings.

What can visitors do there?

A network of walking trails follows the lake's shoreline, popular with visitors seeking a quiet encounter with the Jura landscape. The lake is also valued by anglers. The former ice houses can be visited freely and without booking at any time of year.

Frequently asked questions

How big is Lac de Sylans? The lake covers roughly 49.7 hectares, is nearly 2 kilometres long, up to 250 metres wide, and reaches a maximum depth of 22 metres.

Where exactly is it located? In the commune of Le Poizat-Lalleyriat in the Ain department, a few kilometres east of Nantua.

How did the lake form? A landslide in the Nantua cluse blocked the Doye springs, a process now believed to date back to the Middle Ages rather than the 17th century as once thought.

Can visitors see the old ice houses? Yes, the 19th-century ice house ruins are open for free, unrestricted visits throughout the year.

Is the lake a protected area? Yes, it has been a classified site since 1909 and gained additional ENS protection in 2015.